The invention relates to a least square inverse time offset gradient updating method. The method comprises the following steps of establishing a wavelet-field residual-difference objective function under the two-norm significance according to an initial speed model; conducting the forward modeling on a simulated acoustic equation, storing the boundary wave field of a seismic source and the wave fields of all regions at the last moment; reconstructing the wave field of the seismic source in an inverse mode, conducting the back propagation of the wave field of residual waves, obtaining an accompanying state variable, updating the gradient of the single-shot target generalization relative to model parameters through the inverse scattering condition, and judging whether the multi-shot circulating condition is met or not; if yes, repeatedly executing the above steps and accumulating a single-shot target functional gradient; if not, calculating the conjugation gradient direction by utilizingthe target functional gradient; conducting the forward modeling on the wave field of the seismic source and the data of a conjugate-gradient wave field, and judging whether the multi-shot circulatingcondition is met or not; if yes, repeating and accumulating the data of each shot of the conjugate-gradient wave field; if not, calculating the iteration step size; updating the offset section and updating the iteration; judging whether the iteration condition is met or not; if not, stopping the iteration and outputting an iteration section; if yes, returning the wave field of a forward modelingstorage boundary.